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Are New Zealand's sheep causing an increase in atmospheric methane?

Recent measurements of various atmospheric trace gases in background tropospheric air have shown that the atmospheric concentrations of several are increasing rapidly. The concentration of atmospheric methane, for example, is increasing globally at the rate of 1.7% per year and extrapolation leads to an estimated "Greenhouse Effect" of about 40% of that predicted for increasing atmospheric carbon dioxide. In addition, increasing atmospheric methane will disturb the atmospheric chemistry of the troposphere with unknown consequences for other trace species in the atmosphere. The causes of the increasing atmospheric methane concentration in the atmosphere are not understood. However the suggested sources of the gas, for example biological, do have significantly different stable and radioactive isotopic signatures. A feasible analysis technique based on these isotopic differences will be outlined and the possible atmospheric significance of increasing atmospheric methane will be discussed.